Question

We begin with the surmise that the nucleophilic substitution reactions can occur only by one of two mechanisms: Sn1 solvolysis: three-step mechanism R-X slow, R® + to R® + ROH fast , ROM R' H. R-O + R'OH fast , ROR' + R- SN2: one-step mechanism R-X + Nuc slow, Nuc-R + x Begin by answering the three questions below. If you don't understand what's up, please ask! 1. Write out the rate law for the above Sn1 reaction and the rate law for the above Sn2 reaction. 2. Which mechanism will be favored by: a. a high concentration of a good nucleophile? b. a very polar solvent? c. a highly accessible carbon bearing the leaving group? 3. We wish to test the effect of structure upon reactivity in these two mechanisms. The two reagents we shall use are Nal in acetone and AgNO3 in ethanol. Which reagent for which mechanism? Why? Note that iodide ion is a good nucleophile and nitrate ion is not and that ethanol is a more polar solvent than acetone.

Answer 1

1. Since we know by the concept of Chemical kinetics that slowest tep is the rate determining step. Hence, rate law would depend on Abw sleps only; Sar k[R-x] SA? e-k [R-x] [Nu] ki rate constant ; r = rate of reaction 2. a) SN² reaction depends on none of nucleophile as seen by rate law expression above, high [Nu ] = favouring of ENT b) In SH' reaction generation of carbocation takes place which" is stabilised by interacting with Polar solvent like ethand due to presence of holar bond. BNT reaction is favoured. A good teaving groefs always favours SN2] becaux SN² defends on ease of removal of nucleophile and attack of encoming nucleohhile. An accessible carken readily removes leaving group so that attack of incoming nucleophile can take place. 3) As espained above in and © that holar solvent enhances SN i ethand (Agnez) being highly holar is used for EN .. while NaI (I-) is a good leairing group. Na Il Acetone for [SN²T nchal nucleophile. An awing grauka